1. Field of the Invention
This invention relates to protection of heated metal surfaces of a plasma processing chamber. More particularly, it relates to a plasma processing chamber having metal electrodes covered with particular ceramic materials and shapes to protect the metal electrodes from attack by gaseous species formed in the plasma during processing of a semiconductor substrate and a method of operating such a plasma processing chamber while protecting such metal electrodes from attack by such gaseous species.
2. Description of the Related Art
Critical metal components which may be exposed to chemically aggressive environments in a plasma processing chamber during the processing of heated semiconductor substrates therein include, for example, the susceptor (wafer support) electrode and the showerhead (gas distribution) electrode. Aluminum is generally preferred as the metal for such electrodes since it is a relatively low cost material and the most widely accepted metal for semiconductor processing.
The conventional wisdom, for protecting such aluminum electrode surfaces in plasma enhanced chemical vapor deposition (PECVD) or etching apparatus, is to use a relatively inert ceramic material, such as aluminum oxide anodization, as a protection layer bonded to the aluminum metal surfaces which may be heated to temperatures of about 200-500.degree. C. during such processing of semiconductor substrates. Such anodization sustains high temperature and the aggressive glow discharge excited halogen species which are typical of the required process conditions used, for example, in etching processes.
Other forms of coating of chamber walls or electrodes in plasma processing apparatus are also known. For example, Levenstein et al. U.S. Pat. No. 4,427,516 describes the coating of top plates of wafer-holding assemblies with silicon to minimize contamination during etching. LaPorte et al. U.S. Pat. No. 4,491,496 discloses the coating of metal surfaces of an etching chamber by flame spraying Al.sub.2 O.sub.3 and the coating of the lower electrode (which is at RF potential) with either Al.sub.2 O.sub.3 or tungsten. Fujiyama et al. U.S. Pat. No. 4,526,644 teaches the coating of a stainless steel structure with a metal which does not readily form a fluoride when exposed to a fluorine-containing plasma. Sharp-Geisler U.S. Pat. No. 4,612,432 discloses coating an Aluminum RF electrode with a consumable layer of silicon to avoid contamination of a wafer by sputtering of the aluminum by the plasma. In Japan Document 56-87667, electrode surfaces are coated with carbon or organic materials such as Teflon or polyester to prevent formation of a deposit on a silicon sample during etching. In Japan Document 62-47131, the outer periphery of a wafer support electrode is coated with an insulator in an RIE apparatus to prevent an electric field from concentrating at the periphery of the wafer.
However, aluminum (the material of choice in construction of plasma processing equipment), has a much larger thermal expansion coefficient than most suitable protective coating materials, including ceramics such as aluminum oxide. Consequently, ceramic protection layers (having characteristically low tensile strength) fail by cracking as the heated and intimately bonded aluminum metal expands with increasing temperature. Thus, even when coated or anodized aluminum components are formed using best known manufacturing methods, they eventually fail in such aggressive environments, especially when thermal cycling is required, resulting in both expense and undesired down time for replacement of the failed components.
Prior art attempts at solving this thermal expansion problem have included using low thermal expansion coefficient metal alloys such as iron-nickel-cobalt alloys, e.g., KOVAR as a substitute for aluminum. However, the iron, nickel, and cobalt contained in such an alloy pose a contamination risk in semiconductor device processing. Hazano et al. U.S. Pat. No. 4,693,777 discloses the use of linings over the inner surface of the stainless steel walls of an etch chamber to prevent iron or nickel contamination. The linings are preferably made of aluminum, but may be carbon or silicon carbide. Japan Document 62-113761 describes lining the inner surface of a vacuum vessel with a carbon film to form a slightly gas emitting surface which suppresses the emission of gas from the vessel as a whole. However, with any of these approaches, when aluminum is not used as the chamber and electrode material, it is difficult to guarantee that the iron, nickel, chromium, or cobalt usually found in the substitute materials will never migrate to semiconductor regions where defects would be formed.
It would, therefore, be highly desirable to be able to use a compatible metal such as aluminum as the electrode material in plasma processing apparatus, while providing a surface thereon which is capable of withstanding attack by aggressive species generated in the plasma without incurring the prior art thermal mismatch between such aluminum electrodes and protective ceramic surfaces thereon.